Exploring Photoswitchable Properties of Two Nitro Nickel(II) Complexes with (N,N,O)-Donor Ligands and Their Copper(II) Analogues

Two photoswitchable nickel(II) nitro coordination compounds and their copper(II) analogues are reported. In all these systems, the metal center is chelated by (N,N,O)-donor ligands containing either 2-picolylamine or 8-aminoquinoline fragments. The studied compounds were thoroughly investigated using crystallographic and spectroscopic techniques supplemented by computational analysis. They are easy to synthesize and stable, and all compounds undergo the nitro group isomerization reaction. Nevertheless, there are significant differences between the copper and nickel systems regarding their structural and switchable properties. According to the solid-state IR spectroscopy results, 400–660 nm light irradiation of the ground-state (η2-O,O′)-κ-nitrito copper(II) complexes at 10 K induces a rather moderate conversion to a metastable linkage isomer, which is visible only up to approximately 60–80 K. In turn, upon visible light irradiation (ca. 530 nm excitation wavelength), the ground-state nitro isomers of the examined nickel(II) complexes transform into the endo-nitrito forms. It was possible to achieve about 35% conversion for both nickel(II) systems and to determine the resulting crystal structures at 160 K in the case of single crystals after 30–45 min of exposure to LED light (crystals decayed with longer irradiation), and roughly 95% conversion was achieved for thin-film samples as indicated by the IR spectroscopy results. Traces of the endo-nitrito linkage isomers remained up to 200–220 K, and the isomerization reaction was proven to be fully reversible.


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S27 Figure 15S. Expansion of the 1000−1550 cm −1 region of the Ni-2b complex IR absorption spectra. Black solid line denotes the ground state spectrum, red line presents the spectrum collected after 10 min of LED irradiation, while blue line shows the difference spectrum.
Observations with respect to the computational results (Table 8S) Observations with respect to the computational results (Table 8S): decrease of the bands at 837 and 854 cm −1 corresponding to the δ(ONO) mode of the κ-nitrito configuration and appearance of a new band at 820 cm −1 corresponding to the δ(NO2) mode of the nitro configuration.
The other observable changes correspond to shifts induced by the ONO isomerisation on the other ligand groups.
S29 Figure 17S. Expansion of the 1000−1550 cm −1 region of the Cu-2a complex IR absorption spectra. Black solid line denotes the ground state spectrum, red line presents the spectrum collected after 10 min of LED irradiation, while blue line shows the difference spectrum.
Observations with respect to the computational results (Table 8S) Observations with respect to the computational results (Table 8S): decrease of the band at 837 cm −1 corresponding to the δ(ONO) mode of the κ-nitrito configuration and appearance of a new band at 814 cm −1 corresponding to the δ(NO2) mode of the nitro configuration. S31 Figure 19S. Expansion of the 1000−1550 cm −1 region of the Cu-2b complex IR absorption spectra. Black solid line denotes the ground state spectrum, red line presents the spectrum collected after 10 min of LED irradiation, while blue line shows the difference spectrum.
Observations with respect to the computational results (Table 8S)